Roller driving process and apparatus

ABSTRACT

A roller has a rotary hollow cylinder through which extends a non-rotary crosshead. The hollow cylinder is supported on the crosshead by two opposite rows of hydrostatic supporting elements. When the roller is operated at particularly low linear pressures in the roll gap, a uniform linear force over the whole width of the paper web is exerted in the row of supporting elements opposite to the roll gap, causing an increase in the hydraulic pressures required in the row of supporting elements that faces the roll gap, until said pressures reach a range in which perfect operation of the supporting elements is ensured.

BACKGROUND OF THE INVENTION

The present invention relates to a process and apparatus for driving aroller with a rotary hollow cylinder that forms the working rollercircumference, with a non-rotary crosshead that extends through thehollow cylinder lengthwise, leaving a radial distance from the insidecylindrical surface of the hollow cylinder all around, to which externalforces can be transferred at the ends, with a first row of severalhydrostatic supporting elements arranged on the crosshead and resting onit, closely following each other in its lengthwise direction, which canbe controlled individually or in small groups of at most about threesupporting elements, by means of which independent forces directedagainst the roller gap can be exerted against the inside cylindricalsurface of the hollow cylinder, and with at least a second row ofseveral hydrostatic supporting elements arranged on the crosshead andresting on it, following each other in its lengthwise direction, bymeans of which forces directed away from the roller gap can be exertedagainst the inside cylindrical surface of the hollow cylinder, withsupply lines for hydraulic pressure fluid provided in or on thecrosshead for each individual supporting element, and with a controldevice by means of which the pressures in the individual supply linescan be selected independently of one another.

Such rollers are disclosed, for example, in EP 210 388; rows ofsupporting elements act on the hollow cylinder toward different sides inthe plane of effect of the roller, i.e., generally in the connectingplane of the roller (defined by the plane in which the axis of theroller connects with the axis of a counter-roller), so that variableforce profiles can be exerted on the hollow cylinder over its length,and a desired, generally non-uniform linear force profile occurs in theroller gap. This linear force profile is dependent on the forces exertedby the individual supporting elements and the deformation properties ofboth the hollow cylinder and the product. For each individual case, theapplied forces required to achieve a certain linear force profile can becalculated using known methods, such as the finite element method.

The supporting elements in known rollers interact with the crosshead asa piston and cylinder. Each supporting element has its own supply lineand can therefore have pressure fluid applied to it independently of theother supporting elements. In each supporting element, the supply linesin the crosshead open into a pressure chamber assigned to the element,and the supplied pressure fluid presses the supporting element radiallyoutward, so that it rests against the inside cylindrical surface of thehollow cylinder with a contact surface adapted to this insidecylindrical surface. Flat bearing pockets are formed in the contactsurface, which are connected with the pressure chamber via throttlebores. The pressure fluid supplied to the pressure chamber exits intothe bearing pockets via the throttle bores, and therein forms ahydrostatic pressure cushion, by which the supporting elements exerttheir force against the inside cylindrical surface of the hollowcylinder. The pressure fluid furthermore constantly flows outward, overthe edge of the bearing pockets, and forms a liquid film in the edgeregions of the bearing pockets. This prevents metal-to-metal contactbetween the supporting elements and the inside cylindrical surface ofthe hollow cylinder, and permits additional force to be transferred tothe hollow cylinder.

In the aforementioned, known roller, the forces are transferred by meansof the pressure fluid, which act in the supporting element, and theinterstice between the crosshead and the inside cylindrical surface ofthe hollow cylinder is not involved in formation of the linear force,i.e., it makes no difference whether or not it is filled with pressurefluid.

However, the invention also has significance for rollers according to DE38 20 974 C2. In such rollers, the interstice between the crosshead andthe inside cylindrical surface of the hollow cylinder is not interruptedin the circumferential direction by lengthwise seals or similar devices,but rather entirely filled with pressure fluid under a controlledpressure. The individual supporting elements also receive pressure fluidunder a controlled pressure, which can be slightly above or below thepressure in the surrounding interstice. If the pressure is above thepressure in the surrounding interstice, these supporting elements exerta corresponding positive force against the inside cylindrical surface ofthe hollow cylinder. However, if the pressure in the supporting elementsis below the pressure in the interstice, the supporting element inquestion forms a zone that is separate from the otherwise uniformpressure in the interstice. Therefore, a lower force per cross-sectionalunit is in effect here in comparison to the surrounding interstice. Sucha supporting element therefore delimits a "hole" in the uniform pressureeffect of the pressure fluid in the interstice and has approximately thesame effect as a supporting element acting with a positive effect, atthe pressure difference, if it were affixed on the diametricallyopposite side of the crosshead.

The invention is also suitable for rollers of this type.

Finally, from EP 201 783 A2, the following is known: a roller with arotary hollow cylinder which forms the working roller surface, with anon-rotary crosshead, which extends through the hollow cylinderlengthwise, leaving a radial distance from the inside cylindricalsurface of the hollow cylinder all around, to which external forces canbe transferred at the ends, with a first row of several hydrostaticsupporting elements arranged on the crosshead and resting on it, closelyfollowing each other in its lengthwise direction, separately controlledin groups of up to about three supporting elements, by means of whichindependent forces directed against the roller gap formed between theroller and the counter-roller can be exerted against the insidecylindrical surface of the hollow cylinder, and with at least a secondrow of several hydrostatic supporting elements arranged on the crossheadand resting on it, following each other in its lengthwise direction, bymeans of which forces directed away from the roller gap can be exertedagainst the inside cylindrical surface of the hollow cylinder, withsupply lines for hydraulic pressure fluid provided in the crosshead foreach individual supporting element, and with a control device, by meansof which the pressures in the individual supply lines can be selectedindependently of one another, where at least one group of severalconsecutive supporting elements of the second row is controlled in sucha way that they produce a uniform counter-linear force over the expanseof the group, in the lengthwise direction of the roller.

In this connection, the two rows of supporting elements are referred toas the primary supporting elements, which work toward the roller gap,and the secondary supporting elements. The roller is heated, and heat istransferred to the hollow cylinder by the pressure fluid of the primarysupporting elements. This causes the force exerted by a specific primarysupporting element to deviate. This force deviation is compensated by acorresponding force deviation at the opposing secondary supportingelement. Therefore, the purpose of this known roller is to maintain thelinear force profile where changes in temperature exist.

In some paper treatments, particularly in the last stage of refining,only very low linear forces, in the range of only a few 10 N/cm ofroller length, are desired. It is often difficult, in practice, to bringabout such linear force progressions, because individual supportingelements are allowed to experience only very low pressures, and theremight actually be the theoretical necessity to have "negative" hydraulicpressures in certain supporting elements. Experience has shown, however,that satisfactory operation of hydrostatically supported rollers of thetype in question is possible only at hydraulic fluid pressures that areapproximately 3 bar or higher.

The object of the invention is to exert particularly low linear forcesfor a roller of the type described.

SUMMARY OF THE INVENTION

This object is achieved by providing at least one group of severalconsecutive supporting elements of the second row is controlled in sucha way that they produce a uniform counter-linear force over the expanseof the group, in the lengthwise direction of the roller, which requiresan increase in the forces of the first row to produce a certain linearforce distribution in the roller gap.

The present invention is based on the fact that the linear force exertedoutwardly by the roller results from the difference of the forcesexerted by the two rows of supporting elements. The hollow cylinder is"pulled away" from the roller gap by the supporting elements of thesecond row, at a uniform linear force. In order for the supportingelements directed toward the roller gap to produce the desired positivelinear force--even a small one--they must have a correspondinglyelevated pressure applied to them. Part of this pressure is used tocompensate the forces of the supporting elements of the second row,which are directed away from the roller gap. The linear force thatoccurs in the roller gap results from the portions of the supportingelements of the first row, which are above the uniform linear forcesdirected away from the roller gap. By presetting a uniform linear forcedirected away from the roller gap, the operating pressure of thesupporting elements directed toward the roller gap is artificiallyincreased, specifically into a range that makes perfect operation of thesupporting elements possible.

The supporting elements of the second row can be provided to bediametrically opposite the supporting elements of the first row, andpresent in the same formation and number. However, this is notcompulsory. It is only necessary that the supporting elements of thesecond row produce a force distribution that is uniform and directedaway from the roller gap. This means that there can be fewer supportingelements in a different arrangement than in the first row, as long asthey produce a uniform force in the opposite direction.

The present invention is primarily practical in a case which involveslow linear forces in the roller gap. If the required linear forceprofile in the roller gap is sufficiently high so that forces to beexerted by the individual supporting elements directed toward the rollergap demand sufficiently high hydraulic pressures in the supportingelements, enabling the supporting elements work perfectly, theartificial increases are no longer required and would only represent anunnecessary additional consumption of energy.

The basic effort of the invention is aimed at bringing about fine-tuningof the linear force distribution over the width of the web, withoutusing previously known external aids, such as hot-air jets or inductiveheating devices. The close spacing of the supporting elements, incombination with a sufficiently adaptable hollow cylinder and thestructure of the present invention, results in sensitive adjustment ofthe linear force distribution down to very low linear force values.

The invention has already been described where the uniform linear forcein the second row is present in only one length segment of the roller.

In many cases, however, it will be considered as a possibility tooperate all the supporting elements of the second row at the same linearforce, so that a constant counter-linear force per length unit ispresent over the entire length of the roller.

In the range of lower linear pressures it is particularly important, onthe one hand, but also particularly difficult, on the other hand, tomaintain a certain predetermined linear force progression along theroller gap.

In this range, deviations from the linear force progression byrelatively small absolute force amounts already result in significantrelative deviations and correspondingly high variations in the intendedtreatment effect along the roller gap. In other words, if the treatmentof a paper web, for example, at low linear pressures, is to have thecorrect effect, it is important to accurately ensure the closestpossible adherence to a predetermined linear force progression.

Adherence to a predetermined linear force progression is made moredifficult because the deformation properties of the hollow cylinder and,in particular, the influence of the ends must be taken intoconsideration as interference factors. After all, the hollow cylinder isa finite tube segment, which undergoes a knife-edge load that terminatesat a distance from the ends.

Inside the lengthwise expanse of the hollow cylinder, the latterexperiences an oval cross-sectional deformation by means of thesupporting elements of the two rows, which act in opposite directions,resulting in a certain internal linear force distribution. Towards theends, however, the ovality decreases again, due to the absence of thesupporting elements which act in the same way as on the inside, andnaturally this has an effect on the linear force distribution at theends. For rollers of the type in question, it is a characteristic thateven to achieve a constant linear force progression with a web that iscompletely uniform across its width, a very non-uniform pressuredistribution in the supporting elements located towards the roller endsis necessary. The pressures in the supporting elements at the ends mustbe clearly increased above a mean value present in the center region ofthe roller, and the pressures in the adjacent supporting inward elementsmust also be clearly lowered below this value, in order to achieve auniform linear force distribution. This means that the inherentproperties of the roller already require potential correction near theedge region.

In addition to the internal influences, i.e., those caused by the rolleritself or by its limitation in the lengthwise direction, externalinfluences can also exist in the edge region, and these must also becountered; they are mainly connected with the fact that in most cases,the web to be processed is not uniform over its width.

One of these influences is the temperature profile of thecounter-roller, which is generally warmer at the edge because the paperweb does not conduct off any heat than in inner regions.

In addition, the paper web is usually drier at the edge, which requiresan adjustment of the treatment pressure in order to achieve a uniformtreatment effect.

The paper web might have shrunk in the crosswise direction in the dryingsection of the paper machine, and this as well as the non-uniformtemperature profile makes it necessary to adjust the pressure exerted.

Finally, profile problems of the headbox and the felts can also causethe need for corrections at the edge.

All of these influences must be balanced out by means of corrections atthe edge of the roller, i.e., at the location where corrections arealready necessary due to the roller itself, limiting the remainingcorrection capacity, so that not a lot of correction potential is leftto take external effects into consideration. In this connection, casescan occur in which the roller can no longer correct itself, with its tworows of supporting elements.

A further object of the invention is to improve the correctionpossibilities at the edge for a roller of the type in question.

This task is accomplished where the linear force distribution in theroller gap is additionally corrected at at least one end of the roller.In this way, the linear force distribution in the roller gap isadditionally corrected at at least one end of the roller, i.e., beyondthe correction potential inherent in the roller with its two rows ofsupporting elements, which is acheived, in terms of apparatus, by meansof additional devices to influence the linear force progression at theends of the roller that are provided at the ends of the roller.

The instant invention cannot be viewed just from the aspect of aidingthe roller, so to speak, if non-uniformity of the edge threatens toexhaust the correction potential of the roller itself. Instead, animportant process of using the additional devices consists of balancingout the non-uniformity inherent in the roller right from the start,assuming that the web will subject it to uniform stress. The supportingelements will no longer have to exert greatly different forces at theedge, just in order to balance out the shape-related deformationcharacteristics of the hollow cylinder, but should rather be freed fromthis and be able to exert essentially the same forces over the width ofthe web. If non-uniformities occur at the edge during operation, thuscreating a need for correction, the need can be met entirely by means ofhydraulics, by suitable control of the supporting elements, which are nolonger under stress due to the end correction of the hollow cylinder.Hydraulic correction by the supporting elements themselves can proceedrelatively simply, and, in particular, without delay.

The additional devices can be implemented, in concrete manner, in atleast three different ways, where the various characteristics cancertainly also be present simultaneously in one roller.

In a first known apparatus of a roller with a corresponding structure,the uniform counter-linear force is provided only inside the lengthwiseexpanse of the roller, and two forces "directed backwards" are appliedat the ends in the second row, independently of this force, in order tobring about a desired end correction, as it is actually known from DE-PS23 25 721.

In the second known apparatus of this type, additional elements, whichact against the inside cylindrical surface of the hollow cylinder arealso provided; these are spreading elements acting on both sides of theplane of effect and exerting forces that act on the inside cylindricalsurface of the end of the hollow cylinder perpendicular to the plane ofeffect; they pull the end of the hollow cylinder apart, so to speak,perpendicular to the plane of effect, causing the hollow cylinder to bepulled away in the roller gap, at the end of the roller gap.

The second known apparatus is disclosed in the company brochure ofKleinewefers GmbH "Das Hydrein-Walzsystem" The Hydrein Roller System!and the corresponding DE 33 25 385 C2.

A third known apparatus for influencing the end of a roller according tothe invention provides for additional means for thermal profiling of theend of the hollow cylinder, for example for heating, in order to bringabout an increase in diameter and an increase of the linear force in theroller gap, or for cooling, for the opposite effect.

Preferably, the means can comprise a device for applying a fluid heattransfer medium only to the end region of the inside cylindrical surfaceof the hollow cylinder.

This third known apparatus is disclosed in EP 328 503.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings schematically shows exemplary embodiments of the invention.

FIG. 1 shows the principle of the invention for a suitable roller,partially in longitudinal cross-section;

FIG. 2 shows an example of a desired linear force progression;

FIG. 3 and 4 show diagrams of the forces necessary in the individualsupporting elements in order to achieve the linear force progressionaccording to FIG. 2;

FIG. 5 and 6 show exemplary embodiments;

FIG. 7 shows a cross-section through a roller with two "second" rows ofsupporting elements;

FIG. 8 shows a cross-section through a roller with spreading elementsacting in the crosswise direction;

FIGS. 9A and 9B show two related diagrams which illustrate the pressuredistribution in the first row of supporting elements which is necessaryto achieve a uniform linear force progression.

DETAILED DESCRIPTION

The roller indicated as a whole as 100 in FIG. 1 is the bottom roller ofa pair of rollers and works together with a roller 10. The paper web 2to be treated in the roller gap 1 is given a pressure treatment at lowlinear force, which has a certain profile 15 over the width of the paperweb 2. This profile might result from non-uniformities of the paper webin the width direction and is shown in the curve of FIG. 2 thatreproduces the progression of the linear force LK (force per centimetersof roller length) exerted in the roller gap 1 by the roller 100(measured over the length of the roller 100 from its left end in FIG.1). The non-uniformities can be, for example, non-uniform moisture,non-uniform gloss, etc., which are to be counteracted by the treatment.In the example, therefore, a reduction, i.e., a type of valley 15', isto be present in the linear force in the right half. However, it isunderstood that a uniform, relatively low linear pressure is alsopossible.

The roller 100 comprises a hollow cylinder 3 with a working cylindersurface 4 and a cylindrical, smooth inside surface 6, through whichroller a crosshead 5 extends lengthwise and experiences external supportforces 7 at its ends. The hollow cylinder 3 leaves a radial distance tothe crosshead 5 all around. On the crosshead 5, a first row 8 of closelyspaced supporting elements 9 is provided on the side facing the rollergap 1, to which pressure fluid can be supplied by means of supply linesin the crosshead (not shown) at different pressures per supportingelement. However, small groups of supporting elements 9, for exampleedge groups of two or three supporting elements 9, can also be acted onin common.

Opposite the row 8, offset in the circumferential direction by 180° C.,a second row 12 of supporting elements 13 is provided, extending overthe entire length of the roller 100, which act away from the roller gap1 and therefore attempt to pull the hollow cylinder 3 away from thecounter-roller 10. Instead of the one row 12 of supporting elements 13,two rows 12' and 12" could also be provided, for example, where it ispractical if supporting elements 13 assigned to each other and belongingto the rows 12, 12' and 12' are located in planes perpendicular to theaxis and the resulting forces of the supporting elements 13 of the tworows 12' 12" are located in the plane of effect of the roller 100, i.e.,opposite the force K₉ in each instance (FIG. 7).

The supporting elements 9, 13 are spaced closely in the rows 8, 12,along the roller 100, i.e., without avoidable gaps in their regions ofeffect, but without touching one another and usually have a width ofabout 150 to 300 mm (seen in the lengthwise direction of the roller 100)while the roller 100 can have a working width in the range of 5 to 10 m.Accordingly, about thirty and more supporting elements 9, 13 can beprovided per row 8, 12. The representation in FIG. 1 is therefore not toscale. In general, the supporting elements 9 and 13 of each row 8, 12have the same structure, relative to one another, and one supportingelement 13 opposes the other supporting element 9, in each instance.

In FIG. 1, each of the supporting elements 9, 13 are diametricallyopposite to each another in the plane of effect. In FIG. 7, however, itis indicated that the supporting elements 13 can also be arranged onboth sides of the plane of effect, on the side facing away from theroller gap, in two rows 12' and 12", which form the same angle αrelative to the plane of effect. In this configuration, the resultantforces of the individual supporting elements 13, 13, which are locatedin the same plane perpendicular to the axis of the roller, lie in theplane of effect but are opposed to the force of the supporting element 9located in the same plane perpendicular to the axis.

To form the desired linear force profile 15 crosswise to the paper web2, each of the supporting elements 9 can be controlled individually. Theindividual supporting elements 9 exert local forces in their lengthregion, which act against the inside cylindrical surface 6 in thedirection of the roller gap 1, and produce a linear force profile 15 inthe roller gap 1 and on the working cylinder surface 4 of the hollowcylinder 3, where the rigidity of the hollow cylinder 3 plays asignificant role in bringing about this profile. The forces to beexerted by the individual supporting elements 9 required to achieve thelinear force profile 15 by no means have to be proportional to thelinear force profile 15 in their progression along the length of theroller.

In FIG. 3, the forces K₉ exerted by the individual supporting elements 9are shown that are necessary to produce the linear force profile 15shown in FIG. 2 if the row 12 of the supporting elements 13 is notpresent or not in operation. In this case, the linear force profile 15would be produced exclusively by the upper supporting elements 9. Itturns out that on both sides of the linear force valley 15', an increasein the force exerted by the supporting elements 9 (indicated by thearrows) is necessary, while a reduction takes place in between. Only inthis way can the linear force valley 15', which is sharply limited onthe sides, be achieved.

For perfect operation, the supporting elements 9 require a certainminimum hydraulic pressure on the order of about 3 bar, which isindicated as K_(f) in FIG. 3 and 4. It turns out that the threesupporting elements 9 located in the region of the linear force profilevalley 15' would have to be operated at a pressure less than K_(f), orin certain instances, would actually have to be negative. The desiredlinear force profile 15 could therefore not be produced with such anarrangement of supporting elements.

FIG. 4 shows an arrangement that corresponds to the roller 100 of FIG. 1and in which the two rows 8, 12 with supporting elements 9, 13 offset by180° C. in the circumferential direction are present; these exert forcesK₉ upward, i.e., towards the roller gap 1, and forces K₁₃ downward.

The supporting elements 13 of the second row 12, facing away from theroller gap 1, are controlled in such a way that they all receive thesame hydraulic pressure and produce a uniform counter-linear force overthe length of the roller. If the linear force profile 15 is to beproduced in the presence of such a counter-linear force, it is necessaryto increase the hydraulic pressures prevailing in the individualsupporting elements 9 of the first row 8, in order to have positivelinear forces in the roller gap 1. In this way, the forces K'₉ andtherefore the hydraulic operating pressures necessary in the individualsupporting elements 9 to produce them are artificially shifted into arange in which all the forces K'₉ lie above K_(f), and perfect operationof the supporting elements 9 is ensured.

In FIG. 1, which illustrates the principle, the row 12 of the uniformlycontrolled supporting elements 13 extends over the entire length of theroller 100.

In concrete terms, such a structure is possible primarily for a roller200 according to FIG. 5, in which the hollow cylinder 3 is mounted onthe crosshead 5 on bearings 14 provided at both its ends.

However, it is also possible to control only a group G of severalconsecutive supporting elements 13 of the second row 12 uniformly, as isreproduced for the roller 300 of FIG. 6. The roller 300 is one with aninternal stroke, with no bearings that are comparable to the bearings 14of FIG. 5 being provided at its ends. The hollow cylinder 3 can be movedin suitable guide devices (not shown) in the plane of effect, i.e.,parallel to the plane of the drawing, crosswise to the crosshead 5, inthe direction of the arrows 16. Here in this example, small groups R oftwo supporting elements 13' are provided in the second row 12, at theends, which are not part of the uniformly controlled group G, butinstead are controlled independently of this group, in order to produceforces that act backwards, i.e., directed away from the roller gap 1.Such a measure can become necessary in order to pull the hollow cylinderaway from the roller gap 1 at its ends and to avoid excessive edgepressure on the paper web 2 there, if necessary.

FIG. 6 shows another possibility for an additional influence on the endregion of the hollow cylinder 3. This involves additional spreaderelements 17, which act in the crosswise direction; their arrangement isevident from FIG. 8, and they act just like the supporting elements 9,13, only in a direction of effect rotated by 90° C. in thecircumferential direction. It is true that the ovality of the hollowcylinder 3 produced by the supporting elements 9, 13 decreases towardsthe end, so that the cross-section of the hollow cylinder 3 comes closerto a circular shape again by itself. However, the accompanying decreaseof the linear force in the roller gap 1 can be supported further by thespreader elements 17. The spreader elements do not necessarily have tobe arranged in a perpendicular plane W (i.e., perpendicular relative tothe plane of effect) as shown in FIG. 8. If the angles α of theembodiment according to FIG. 7 are selected to be larger than shown, acertain spreading effect also results, if a counter-force acting awayfrom the roller gap 1 is applied at the same time.

FIG. 6 also indicates possibilities for a thermal influence on the endsof the hollow cylinder. For example, inductive coils 18 could bearranged at the ends, just outside the hollow cylinder 3. Alternatively,the thermal profiling could also take place internally, which isrepresented by the spray device 19. The effect of the heat carriermedium sprayed against the inside cylindrical surface 6 of the hollowcylinder 3 from the inside is limited to the end region of the hollowcylinder 3, and gives it a different temperature with a correspondingdeviation from the cylindricity and an influence on the linear forceprogression at the end.

FIG. 9A and FIG. 9B serve as a detailed explanation of the specialsignificance of the additional measures described in connection withFIGS. 6 to 8 to influence the linear force profile at the ends of theroller 100. FIGS. 3 and 4 are highly schematic in this respect, and areonly intended to illustrate the principle. FIG. 9A, however, is based onan actual calculation of the hydraulic pressures necessary in theindividual supporting elements 9. Let us assume that the web to betreated has uniform properties over its width W_(B) and extends over thelength of the roller 100. An effort is made to exert a linear force L of100 N/mm of product length on this web, uniformly distributed over itswidth, as indicated by the horizontal line 20. The roller 100 of thisexample has thirty-two supporting elements 9. The force exerted by eachindividual supporting element 9_(i) is proportional to the hydraulicpressure p_(i) which prevails in it (i =1 . . . 32). The diagram in thebottom part of FIG. 9 clearly shows that the distribution of thepressures p_(i), i.e., the individual forces exerted by thecorresponding supporting elements 9_(i), which are required for aconstant progression of the linear force L in the roller gap 1 is farfrom constant at the ends of the roller 100. The supporting elements 9₁,and 9₃₂, which are located at the ends, must receive pressures and exertforces that are significantly above the mean value present in the regionof the center supporting elements 9₁₃ to 9₂₀. The supporting elements 9₂to 9₆ and 9₂₈ to 9₃₁, which are inwardly adjacent to the end supportingelements 9₁ and 9₃₂, on the other hand, must bring about forces whichare significantly below the mean value indicated.

The great non-uniformity of the required exertion of force by thesupporting elements near the edge region is inherently due to theproperties of the roller 100 itself. If external influences are added tothis, and the web is not uniform over the entire length of the roller100, as was assumed above, additional corrections might be necessary atthe edge, i.e., the supporting elements 9₁ and 9₃₂ might have to exerteven higher forces and the adjacent supporting elements might have toexert even lower forces. In this regard, the roller 100 can encountertechnical limits, if it works only with the supporting element rows 8and 12.

In these cases, additional correction devices relating to FIG. 6 to 8are significant. However, the correction devices can also be used insuch a way that they serve to make the forces K'g_(i) demanded from theindividual supporting elements 9 uniform; their non-uniformdistribution, as it is required for roller correction, i.e., to balanceout the deformation properties of the hollow cylinder 3, is shown inFIG. 9B. The correction devices therefore influence the roller 100,without taking into consideration any non-uniformities, in such a waythat the supporting elements 9 can produce an essentially uniform forceeven at the edge, which guarantees uniformity of the linear force in theroller gap. The uniform force distribution is indicated with the brokenline 21.

If the web then has non-uniformities at the edge, the supportingelements located there still have their correction potential available,and the web correction can take place by means of these supportingelements 9, i.e., without the additional correction devices having to beused for this purpose.

What is claimed is:
 1. In a system for driving a roller includinga firstroller having a rotating hollow cylinder and a lengthwise end, thehollow cylinder further including a working cylindrical surface and aninside cylindrical surface, a second roller with a roller gap disposedbetween the first roller and the second roller, a non-rotatingcrosshead, extending lengthwise through the hollow cylinder, radiallyspaced from the inside cylindrical surface, and receiving externalsupport at the lengthwise ends of the crosshead, a first row of aplurality of hydrostatic supporting elements, each of the plurality ofhydrostatic elements coupled to the crosshead and adjacently disposed toeach other in the lengthwise direction of the first row, wherein each ofthe plurality of supporting elements of the first row are controlled ingroups of approximately no more than three supporting elements toproduce independent forces that are directed toward the roller gap andexerted against the inside cylindrical surface, a second row of aplurality of hydrostatic supporting elements, the second row having asubgroup of a plurality of adjacent supporting elements, each of theplurality of hydrostatic elements of the second row coupled to thecrosshead and adjacently disposed to each other in the lengthwisedirection, wherein the second row of hydrostatic supporting elements arecontrolled to produce forces directed away from the roller gap that areexerted against the inside cylindrical surface, a plurality of hydraulicfluid supply lines disposed in or on the crosshead, and a device forindependently controlling each of the plurality of hydraulic fluidsupply lines,a process for driving the roller comprising the step of:controlling the pressure in the subgroup of the second row to produce auniform counter-linear force in each of the supporting elements of thesubgroup, wherein forces in the supporting elements of the first row andthe forces in the supporting elements of the subgroup are above theminimum force corresponding to the minimum pressure necessary for idealoperation of the supporting elements.
 2. The process according to claim1, wherein the linear force distribution in the roller gap is correctedat the lengthwise end of the first roller.
 3. A apparatus for driving aroller comprising:a first roller having a rotating hollow cylinder and alengthwise end, the hollow cylinder further including a workingcylindrical surface and an inside cylindrical surface, a second rollerwith a roller gap disposed between said first roller and said secondroller, a non-rotating crosshead, extending lengthwise through thehollow cylinder, radially spaced from the inside cylindrical surface,and receiving external support at the lengthwise ends of said crosshead,a first row of a plurality of hydrostatic supporting elements, each ofthe plurality of hydrostatic elements coupled to the crosshead andadjacently disposed to each other in the lengthwise direction of thefirst row, wherein each of the plurality of supporting elements of thefirst row are controlled in groups of approximately no more than threesupporting elements to produce independent forces that are directedtoward the roller gap and exerted against the inside cylindricalsurface, a second row of a plurality of hydrostatic supporting elements,the second row having a subgroup of a plurality of adjacent supportingelements, each of the plurality of hydrostatic elements coupled to thecrosshead and adjacently disposed to each other in the lengthwisedirection, wherein the second row of hydrostatic support elements arecontrolled to produce forces directed away from the roller gap that areexerted against the inside cylindrical surface, a plurality pf hydraulicfluid supply lines disposed in or on the crosshead, and a device forindependently controlling each of the plurality of hydraulic fluidsupply lines, wherein the device controls the pressure in the subgroupof the second row to produce a uniform counter-linear force in each ofthe supporting elements of the subgroup, and forces in the supportingelements of the first row and the forces in the supporting elements ofthe subgroup are above the minimum force corresponding to the minimumpressure necessary for ideal operation of the supporting elements.
 4. Aapparatus according to claim 3, further comprising additional devicesfor influencing the linear force progression near the ends of the firstroller, said additional devices being disposed near the ends of thefirst roller.
 5. A apparatus according to claim 4, wherein the pluralityof supporting elements of said second row includes a first portion ofsupporting elements near the ends of said second row and a secondportion, said first portion being controlled independently of the secondportion.
 6. The apparatus according to claim 5, further comprisingspreading elements which act towards both sides of the plane of effectare provided at the ends of the roller, which exert forces which act onthe inside circumference of the ends of the hollow cylinder, crosswiseto the plane of effect.
 7. The apparatus according to claim 6, whereinmeans for thermal profiling of the end of the hollow cylinder areprovided at the ends of the roller.
 8. The apparatus according to claim7, wherein the means for thermal profiling comprise a device forapplying internally a fluid heat carrier medium only to the end regionof the inside circumference of the hollow cylinder.
 9. The apparatusaccording to claim 5, wherein means for thermal profiling of the end ofthe hollow cylinder are provided at the ends of the roller.
 10. Theapparatus according to claim 9, wherein the means for thermal profilingcomprise a device for applying internally a fluid heat carrier mediumonly to the end region of the inside circumference of the hollowcylinder.
 11. The apparatus according to claim 4, further comprisingspreading elements disposed at the ends of the first roller, saidspreading elements exerting forces on the inside cylindrical surfacenear the ends of the first roller in a direction substantiallyperpendicular to the plane of effect.
 12. The apparatus according toclaim 11, wherein means for thermal profiling of the end of the hollowcylinder are provided at the ends of the roller.
 13. The apparatusaccording to claim 12, wherein the means for thermal profiling comprisea device for applying internally a fluid heat carrier medium only to theend region of the inside circumference of the hollow cylinder.
 14. Theapparatus according to claim 4, wherein means for thermal profiling ofthe end of the hollow cylinder are provided at the ends of the roller.15. The apparatus according to claim 14 wherein the means for thermalprofiling comprise a device for applying internally a fluid heat carriermedium only to the end region of the inside circumference of the hollowcylinder.